EP3230833A1 - Method for continuously detecting a state contact or non-contact between a tip of an instrument and a writing medium having a variable thickness, and related system - Google Patents

Abstract

The invention relates to a method for continuously detecting a contact state or non-contact state between a tip (P) of an instrument (U) and a writing surface (SE) of a writing medium (SUE) which has a variable thickness and is arranged on a bearing surface (SA) of a tracking device (DL).

Description

 Method for continuously detecting a state of contact or non-contact between a tip of a utensil and a writing medium of variable thickness, and associated system

The present invention relates to a method for the continuous detection of a state of contact or non-contact between a tip of a utensil and a writing medium of variable thickness, and associated system. continuous state of contact or non-contact between a tip of a utensil and a writing medium of varying thickness.

It is known to use a writing implement equipped with a contact detection device such as a pressure sensor, but such a writing utensil needs to be equipped with an electronic device using a power source. .

It is also known to use a touch-sensitive writing surface, such as a tactile interface, but such an approach does not make it possible to use passive supports of variable thickness, such as a notebook.

It is also known to use a fixed contact threshold, for example as described in document FR 2988874, which does not make it possible to overcome the errors in estimating the position of the tip of the writing utensil, requires a submillimetric estimation error, and is not effective if the writing medium is deformed by the touch of the utensil.

An object of the invention is to overcome these problems.

According to one aspect of the invention, there is provided a method of continuously detecting a state of contact or non-contact between a tip of a utensil and a writing surface of a thick writing medium. variable disposed on a support surface of a locating device; said method comprising the steps of: determining, at a current time, the position of the tip of the utensil relative to the bearing surface;

 determining, at the current time, a distance between the bearing surface and the tip, from said position of the tip;

 updating the current value of a minimum distance between the bearing surface and the writing surface with the distance between the bearing surface and the tip at the current time, when said distance is less than the current value the minimum distance;

 determining, at the current instant, a parameter of movement of the tip, representative of a ratio between the movement tangential to the writing surface and the normal movement at the writing surface between the current instant and the instant previous;

 determining, at the current instant, an amplitude parameter representative of the amplitude of the movement of the tip between the current instant and the previous instant;

 determining a state of contact between the tip and the writing surface, at the current time, when:

 the distance is less than or equal to the sum of a current minimum distance and a tolerance, said tolerance being representative of an uncertainty as to the distance between the writing surface and the bearing surface;

the movement parameter is representative of a movement substantially tangential to the writing surface, i.e. its value is less than or equal to a tangential movement threshold; and

 a norm of the amplitude parameter is greater than or equal to a minimum amplitude threshold;

 to determine, otherwise, a state of non-contact between the tip and the writing surface, at the current instant.

Thus, it is possible to estimate the position of the tip of the writing implement, accurately even if the writing medium is deformed to the touch of the utensil. Thus, the invention works as well with a single sheet with a notebook. In one embodiment, a sliding time window is used on which respective weighted averages are used for said position, and / or said distance, and / or said motion parameter, and / or said amplitude parameter; the sliding window comprising a set of data samples corresponding to a set of successive instants in which a current instant is considered, the other instants of the set preceding the current instant being considered as past instants of the sliding window, and the other instants of the set succeeding the current instant being considered as future instants of the current sliding window.

In other words, on a current sliding window, comprising a plurality of successive instants, the current instant considered is not the last.

This improves the accuracy of the process, which allows a better visibility on the nature of the movement, and thus reduce false positives. According to one embodiment, the method further comprises a step of updating said tolerance when two successive determinations of contact or non-contact state determine the same state. Thus, the method adapts better to the state of the writing surface without losing the reactivity during rapid loss of contact.

In one embodiment, when two successive state determinations are in contact, said tolerance is reduced without being less than a minimum tolerance threshold.

Thus, reducing the tolerance allows the process to be responsive to rapid contact losses. According to one embodiment, when the motion parameter is representative of a movement that is essentially tangential to the writing surface, ie its value is less than or equal to the tangential motion threshold (it would be desirable to keep the same, because the same condition is defined) and the distance is greater than the sum of the current minimum distance and the tolerance, the current minimum distance is increased.

Thus, increasing the minimum distance allows the method to follow a possible change in the thickness of the support depending on the contact position, which is convenient for the case of media whose contact surface is not of regular height.

In one embodiment, when two successive state determinations are non-contact and the value of the motion parameter is less than a maximum movement threshold, said tolerance is increased without being greater than a maximum tolerance threshold.

According to one mode of implementation, the utensil is provided with a fixed magnetic element and the localization device used is provided with an array of at least N triaxial magnetometers connected to each other mechanically without any degree of freedom to preserve a known relative position of these magnetometers, N being an integer at least equal to 2.

Thus, it is possible to use passive writing media (paper, notebook, notebook ...) with a passive utensil (a simple stylus ...) to which is attached a magnet.

It is also proposed, according to another aspect of the invention, a system for continuously detecting a state of contact or non-contact between a tip of a utensil and a writing surface of a writing support. variable thickness disposed on a bearing surface of a locating device; said system comprising a computer adapted to:

 - Determine, at a current time, the position of the tip of the utensil relative to the bearing surface;

- determining, at the current time, a distance between the bearing surface and the tip, from said position of the tip; updating the current value of a minimum distance between the bearing surface and the writing surface with the distance between the bearing surface and the tip at the current time, when said distance is less than the value current of the minimum distance;

 determining, at the current moment, a parameter of movement of the tip, representative of a ratio between the movement tangential to the writing surface and the normal movement at the writing surface between the current instant and the previous moment;

 determining, at the current instant, an amplitude parameter representative of the amplitude of the movement of the tip between the current instant and the preceding instant;

 - determining a state of contact between the tip and the writing surface, at the current time, when:

 the distance is less than or equal to the sum of a current minimum distance and a tolerance, said tolerance being representative of an uncertainty as to the distance between the writing surface and the bearing surface;

the movement parameter is representative of a movement substantially tangential to the writing surface, i.e. its value is less than or equal to a tangential movement threshold; and

 a norm of the amplitude parameter is greater than or equal to a minimum amplitude threshold;

 - Otherwise, determine a state of no contact between the tip and the writing surface at the current time.

The invention will be better understood by studying a few embodiments described by way of non-limiting examples and illustrated by the appended drawings in which:

 - Figures 1 to 10 schematically illustrate a method according to one aspect of the invention.

In all the figures, the elements having identical references are similar. The method and system of the invention assumes to have at any time an estimate of the position of the tip P of a utensil U, such as a stylus or pen, relative to a reference plane or bearing surface SA on which is placed the writing medium SUE comprising a writing surface SE. The writing medium SUE, for example a notebook or notebook, can more or less be deformed. The object of the invention is to make it possible to decide whether at a moment the tip of the utensil is in contact with the writing medium or not.

A reference (Ο, Χ, Υ, Ζ) bound to the reference surface is defined, such that the X and Y axes are parallel to the writing surface SE and the Z axis is perpendicular to this surface SE. The position Pos of the tip P of the utensil U at the instant is described in this reference: The x and y components along the X and Y axes are called tangential components and are represented by the vector t (ti). The component z along the Z axis is called the normal component. We call h the distance between the tip P and the bearing surface SA (h = || z ||). For example, the utensil U may be provided with a magnetic element

Fixed EM and the locating device DL used to be provided with a network R of at least N triaxial magnetometers M, mechanically connected to each other without any degree of freedom to maintain a known relative position of these magnetometers Μ ,, N being a integer at least equal to 2. Similar utensils U and DL locating device are for example described in documents FR 2988862 and FR 2988874.

However, the invention also applies to a digitization system based on the ultrasonic localization in which an ultrasonic transmitter is connected to the utensil and an ultrasonic reception system is connected to the writing surface. The invention also applies to a system of optical location using a camera that can determine the position of a point of the utensil in three dimensions.

In the remainder of the description, the method is described using a sliding window using n successive values, but it can also be applied without a sliding window, ie on a single datum at a time (n = 1), which amounts to considering a sliding window comprising only one data sample at a time. The last n estimates of the position of the tip P of the utensil U are placed in a sliding window:

Pos (ti_ _{n + 1} ) ... PosCti.J Pos (ti) (2)

The decision instant t-G ti_ _{n + 1} ti is defined as the moment for which the decision is made (contact or non-contact). The instants t _k such that t _{ . ^■ n + 1 ≤-t ^L _V k <ti

The instants t _k such that t _j <t _k <ti are called future instants.

A motion vector at time ti is defined as: dPos (ti) = Pos (ti) - PosC i) (3)

As for positions, we form a sliding window of movements at successive moments:

dPos (ti_ _{n + 2)} ) ... dPos (ti)

A motion can be composed of a tangential motion dt (ti) corresponding to the two axes X and Y, and of a normal movement dz (ti): dx (ti) ^" x (ti) -x (ti- -l)

 dPos (ti) = dy (ti) = y (ti) -y (ti.-1) (4)

dz (ti) Z (ti) - z (ti_

 -l)

We call the amplitude of motion at time t _{ the scalar value HdPos (ti) ll (5)

A motion parameter at time t _{ (denoted PM (ti)) makes it possible to distinguish whether the movement is rather tangential with respect to the writing surface or rather normal to the writing surface.

The motion parameter does not give useful information if the range of motion is small. This is due to the noise on the estimate of the Pos position of the tip P (and thus on the movement). It is therefore necessary to link the tests on the movement parameter to a test on the range of motion to have certainty on these tests.

One can for example use as an indicator the ratio of the normal movement by the norm of the movement:

The value of this parameter is between -1 and 1. The value 1 corresponds to a movement that is entirely normal to the writing surface SE (i.e. when the tip P moves away from the writing surface SE). The value 0 corresponds to a movement perfectly parallel to the writing surface SE. The value -1 corresponds to a movement that is entirely normal to the writing surface SE when the tip P approaches the writing surface SE.

The ratio of the absolute value of normal movement (or distance h) to the norm of movement can be used as the movement parameter:

PMCt = ^{| dz (ti) l} = ^{dh (ti)} ( ₆ )

^ ^lJ || dPos (ti) || || dPos (ti) || '

This form is easy to compare to a threshold since the result is included between 0 and 1. The value 1 corresponds to an entirely normal movement on the surface. The value 0 corresponds to a movement perfectly parallel to the writing surface SE (or more precisely the support surface SA).

It is also possible to use the value of the normal component of the movement dz (ti) or the variation of the distance dh (ti), which gives the amplitude of the movement in the normal direction. Generally, when the tip P of the utensil U is in contact with the writing surface SE, this component does not vary much over time. On the other hand, when the utensil U is not in contact with the writing surface SE, it is difficult to keep it at a low value. For greater certainty, it is also better to require that the amplitude dPos (ti) of the movement exceeds a threshold dPos _mir , to determine the nature of the movement.

A sliding window containing the values of the motion parameter ΡΜ (ΐ,) is formed at various successive instants:

Two other parameters are estimated at each reception of a new estimate of the position Pos (ti) of the tip P of the utensil U: the minimum level of the contact surface at the moment of decision hmin (tj)

The tolerance tol _h (t _j ) on the level of the contact surface or writing surface SE at the time of decision (the uncertainty on the level of the contact surface).

Figure 1 shows the various parameters describing the writing surface SE. Finally, it is possible to define a Boolean variable CONTACT which describes the state of contact between the tip P of the utensil U and the writing surface SE. The following description describes in detail a non-limiting embodiment of the present invention.

Initially, the minimum level of the contact surface, or, in other words, the current value of the minimum distance h _min (ti) between the bearing surface SA and the writing surface SE of the writing medium SUE hmin (ti) is unknown. It is initialized to the maximum tolerable value of the thickness of the writing medium, for example 3 cm, considering that this is the maximum thickness of a notebook.

The tolerance is set at an initial value, for example 1 mm, and the tip P of the utensil U is considered to be in a non-contact state. It is then expected to have received n estimates of the position of the tip P of the utensil U. The iterative processing of the method, as illustrated in FIG. 3, comprises the following steps:

 at each new estimate of the position of the tip P of the utensil U, update the values of the parameters taken into account in the sliding window (Pos, h, PM, dPos, you);

- make a preliminary estimate of the minimum distance h _min (t _j ) between the bearing surface SA and the point P; and

determining a state of contact or non-contact between the tip P of the utensil U, and optionally refining the estimate of the minimum distance h _min (t _j ) between the bearing surface SA and the tip P and / or the estimate of tolerance tol (t _j ).

Concerning the updating of the sliding window, at a time of estimation t _i; we have the following values: Pos (ti_ _{n + 1} ); ... PosCt ^); Pos (ti) (8)

[dPos (ti_ _{n + 2)} ); ... dPosCti.J; dPos (ti)] (9)

[PM (ti_ _{n + 2)} ) ... PM (ti)] (10)

Contact (t _J _ ₁ ) (true or false)

The method of the invention aims at estimating h _min (t _j ), tol (t _j ) and Contact (t _j ), with t _j G pi- _n + i; ¼]. These parameters depend on the contact state (boolean respectively 1 for a contact state and 0 for a non-contact state) at the previous instant (t ^) as well as the value of the various calculated parameters.

We begin with a preliminary estimate of the minimum level of the contact surface, or minimum distance h _min (t _j ) between the bearing surface SA and the tip P, at time j. This estimate can be calculated in several ways depending on the number of points recorded as well as the position of the decision instant in the window. For example, we can compare h _min (t _j ) to the vertical distance

(h (t _j )) of the tip P of the utensil U at the same time. If h (t _j ) <h _min (t _j ) then h _min (tj) = h (t,).

This comparison can also be made with respect to all the points of the sliding window, the average value, or the median value of the window. It can also be performed with respect to the future instants or at times past, compared to the instant of the sliding window considered as the current time. The use of an average value h _Mean (t _j ) on all or some of the points of the sliding window makes it possible to reduce the effect of the noise: ( _t _Σk _{= l} t _k )

lean W ^" _m _ _{1+ 1} in which:

i- n + l <l <m <i n represents the number of points in the sliding window,

i: index of the first point of the part used for the calculation of the average (median or the minimum value) of the points of the sliding window m: index of the last point of the part of the points of the sliding window m-1 + 1 : number of points in this part

The higher the estimation noise of the pos position of the point P, the more points are needed to limit its effect.

In order not to have a lot of delay (detection of the contact or non-contact state), it is better to reduce the number of points in the sliding window succeeding the decision instant t _j .

The determination of the state of contact or non-contact at time t _j is conditioned by the previous state t _i _ _{1 as} well as all or part of the parameters saved in the sliding window.

Also, we define a machine with two states (contact / non-contact). The transition from one state to another is carried out by tests and thresholds which are adapted to the locating device DL making it possible to estimate the position of the tip P of the utensil U. These thresholds can also be optimized as a function of a style of writing or drawing.

These tests are divided into two categories: mandatory tests that can work with a fairly precise system, and optional tests to fill some imperfections in estimating the Pos position of the tip P as well as the deformations of the writing medium SUE, ie of the writing surface SE.

The tolerance tol _h (t _j ) on the level of the writing surface SE can be either fixed (adapted in cases where the device for locating the estimation of the position of the tip of the utensil is fairly accurate and that the support does not deform too much) is variable, in which case, its estimate will be refined according to the test results. Passing conditions from non-contact state to contact state include the following mandatory conditions:

- the vertical position of the point P, or in other words the distance h (t _j ) between the bearing surface SA and the point P, at time t _j is below the minimum level h _min (t _j ) of the contact surface or writing surface SE to which is added the tolerance on the level of the contact surface SE.

the movement parameter PM (t _j ) of the tip P indicates a movement parallel to the writing surface SE. In addition, it is necessary to couple this condition to a minimum condition (threshold) on the parameter of amplitude of movement || dPos (t _j ) || > _min .

These two conditions must be realized to go from the non-contact state to the contact state. The amplitude of motion dPos (t _j ) and the motion parameter PM (t _j ) can be calculated only on the decision instant (t _j ) or on several instants of the sliding window, ie by making combinations on future instants and past instants of the sliding window (for example, maximum value, minimum value, average value or median value). For example, this makes it possible to detect the moment of passage between vertical movement and horizontal movement as shown in FIG. 4, which generally corresponds to the moment of contact between the tip P and the writing surface SE. The movement is essentially normal to the writing surface

SE before the start of writing, then it is tangent to the surface during rewriting. One can, for example, calculate a weighted average of this parameter PM on the future instants and determine if it corresponds to a movement tangent to the surface or not. An additional optional condition may be to perform a low-pass filtering of the position of the tip P or only of the normal component Z of the position, it is possible that the rapid movements, such as the inscription of a point, do not verify not the mandatory conditions. In this case, we add an optional condition to handle this problem. Figure 5 represents this case.

If the tip P of the utensil U makes a downward movement followed by a rising movement, and the position Pos of the tip P at the decision time is below a maximum threshold of distance to the surface of SA support provided for this option, so we go to the contact state for a single iteration.

To detect this, one can use the motion parameter at the instant or the previous instants and the motion parameter at the instant or the moments following, and detect the moment when one has a fast descent followed by a fast rise.

It is also possible to resort to combinations, for example linear combinations, past times and combinations, for example linear combinations, future times to detect the moment of contact between the tip P of the utensil U and the writing surface, for example, calculating the average, the median value ... Regarding the refining of the tolerance you on the level of the contact surface SE to the non-contact state, if none of the contions is checked, we remain in the state of non-contact. Incidentally, it is possible to refine the tolerance in order to fill certain problems such as the imprecision of the locating device DL of the tip P, the bending of the paper during rewriting, and the increase of the thickness of the writing medium SUE (the decrease being managed by the preliminary estimate of the level of the writing surface SE).

If the movement parameter PM makes it possible to detect a tangential movement such that the point P is below a maximum threshold, the tolerance is incremented. This technique makes it possible to quickly reach the new thickness of the writing medium SUE and not to lose a lot of points because of such a change. The result can therefore be very satisfactory but depends on the acquisition frequency of the location device. This is illustrated in Figure 6.

It is also possible to save the instants passed as long as the movement is rather tangential and as soon as the tolerance reaches the new thickness, one can correct the decisions of these past instants by considering that there was contact. This makes it possible to catch up in the event of significant changes in the thickness of the support, and is useful when the deformation of the support is very important or when the system is not precise enough. Conditions for switching from contact state to non-contact state include the following mandatory conditions:

- the vertical position of the point P, or, in other words the distance h (t _j ) between the bearing surface SA and the point P, at time t _j is above the minimum level h _min (t _j ) of the writing surface SE to which the tolerance is added on the level of the contact surface SE.

the movement parameter PM (t _j ) of the tip P indicates a movement having a slope with respect to the writing surface SE greater than a threshold. Depending on the motion parameter used, it is necessary to couple this condition to a minimum condition (threshold) on the amplitude parameter of motion || dPos (t _j ) || > _min .

FIG. 7 represents a simple case of end of contact between the tip P of the utensil U and the writing surface SE. These mandatory conditions do not make it possible to mitigate problems related to the bending of the writing medium SUE as well as the defects of estimation of the position of the tip P. Also, it is possible to combine additional optional conditions to determine if the tip P is well disconnected from the writing surface SE or it is just a bending of the writing surface SE, a change of thickness due to the fact that the writing surface SE is not perfectly parallel to the xy plane of the support surface SA, or a problem related to the accuracy of the acquisition device DL. Such a case is illustrated in FIG.

These optional conditions are based on the PM motion parameter (or a combination of the motion parameter values at past and / or future times). Indeed, the signature of survey of the tip P during a writing or a drawing and that of a change of thickness are not different, and it suffices to apply a double thresholding:

 a more tolerant thresholding at the level of the mandatory condition using the movement parameter PM, and

 a stricter thresholding at the level of the optional conditions which makes it possible to go into the non-contact state only if an upper slope movement has been detected. In the opposite case, refinements of the estimation of the two following parameters are carried out:

the minimum level of the contact surface at the moment of decision h _min (t _j ),

the tolerance on the level of the writing surface at the decision time tol _h (t _j ) (the uncertainty on the level of the writing surface)

Thresholding means comparing the PM motion parameter with a threshold. If the latter does not exceed the threshold, the parameter estimate is refined. This threshold makes it possible to say that, probably, the thickness of the support has changed, and that there is always contact. As with other conditions related to PM motion indicators, it is best to pair it with an amplitude of motion test for best results.

Concerning the refining of the minimum distance estimation h _min (t _j ) between the bearing surface SA and the writing surface SE in the state of contact. If none of the optional conditions are satisfied, it remains in the contact state. Incidentally, it is possible to refine the estimate of the minimum level h _min (t _j ) of the writing surface SE. This makes it possible to adapt to changes in thickness during writing, for example if there are small bumps on the writing surface SE of the support SUE or if the support SUE is not perfectly parallel to the plane xy.

If the position Pos of the tip P of the utensil U, ie the distance h (t _j ) exceeds the minimum level h _min (t _j ) of the writing surface to which is added the tolerance tol (t _j ) on the level of the writing surface SE, the value of the minimum level h _min (t _j ) of the writing surface SE is increased. This is valid up to a certain threshold so as not to favor false positives or false contact detections. Thus, according to the thresholding parameters as well as the increment values, the solution is more or less fault-tolerant at the level of the writing surface SE.

Figure 9 illustrates a refinement of the tolerance on the level of the writing surface in the state of contact. When the mandatory conditions are not verified, it is clear that the tip P is in contact with the writing surface SE. In this case, it may be possible to take advantage to reduce the tolerance tol (t _j ) on the level of the writing surface SE. The tolerance tol (t _j ) can not fall below a minimum threshold in order to avoid problems with holes in the lines drawn because of imperfections.

This operation makes it possible to quickly detect contact losses, since when this interval reaches its minimum value, a lifting of the tip P quickly removes it from the tolerance zone and makes it possible to validate the mandatory conditions proposed above.

Figure 10 illustrates an example of the state machine (contact or non-contact) with the optional update of tolerance tol (t _j ). The steps of the method described above may be performed by one or more programmable processors executing a computer program for performing the functions of the invention by operating on input data and generating output data.

A computer program can be written in any form of programming language, including compiled or interpreted languages, and the computer program can be deployed in any form, including as a stand-alone program or as a subroutine, element, or other unit suitable for use in a computing environment. A computer program can be deployed to run on one computer or multiple computers at a single site or spread across multiple sites and interconnected by a communications network.

Claims

1. Method for the continuous detection of a state of contact or non-contact between a tip (P) of a utensil (U) and a writing surface (SE) of a writing medium (SUE) of variable thickness disposed on a support surface (SA) of a locating device (DL); said method comprising the steps of:  - determining, at a current time (.,), the position (Pos (ti)) of the tip (P) of the utensil (E) relative to the bearing surface (SA);  - determining, at the current instant (.,), a distance ((t,)) between the bearing surface (SA) and the tip (P), from said position (Pos (ti)) of the tip (P);  - update the current value of a minimum distance (hmin (ti)) between the support surface (SA) and the writing surface (SE) with the distance (h (ti)) between the bearing surface (SA) and the tip (P) at the current moment (.,), when said distance (h (t,)) is less than the current value of the distance minimum (h _min (ti));  determining, at the current instant (t,), a motion parameter (ΡΜ (ΐ,)) of the tip (P), representative of a ratio between the movement tangential to the writing surface (SE) and the normal movement at the writing surface (SE) between the current instant (t,) and the previous instant (t, i);  determining, at the current instant (t,), an amplitude parameter (dPos (ti)) representative of the amplitude of the movement of the tip (P) between the current instant (t,) and the instant previous (ti-i);  determining a state of contact between the tip (P) and the writing surface (SE) at the current instant (t), when: - the distance ((t,)) is less than or equal to the sum of a current minimum distance (h _min ) and a tolerance (you (t,)), said tolerance being representative of an uncertainty on the distance ((t,)) between the writing surface (SE) and the bearing surface (SA); the motion parameter (ΡΜ (ΐ,)) is representative of a movement essentially tangential to the surface writing (SE), ie its value is less than or equal to a tangential motion threshold (SMT); and  a norm of the amplitude parameter (dPos (ti)) is greater than or equal to a minimum amplitude threshold (dPOSmin);  - Otherwise, determine a non-contact state between the tip (P) and the writing surface (SE) at the current time (.,). The method according to claim 1, wherein a sliding time window is used on which respective weighted averages are used for said position (Pos (t _j )), and / or said distance (h (t _j )), and / or said motion parameter (PM (tji)), and / or said amplitude parameter (dPos (t _j )); the sliding window comprising a set of data samples corresponding to a set of successive instants in which a current instant is considered, the other instants of the set preceding the current instant being considered as past instants of the sliding window, and the other instants of the set succeeding the current instant being considered as future instants of the current sliding window. The method of claim 1 or 2, further comprising a step of updating said tolerance (t (tj)) when two successive determinations of contact or non-contact state determine the same state. 4. Method according to claim 3, wherein, when two successive state determinations are in contact, said tolerance (you (tj)) is reduced without being less than a minimum tolerance threshold (tol _min ). 5. Method according to claim 4, wherein, when the motion parameter (ΡΜ (ΐ,)) is representative of a movement substantially tangential to the writing surface (SE), ie its value is less than or equal to the threshold of tangential motion (SMT) and that the distance ((t,)) is greater than the sum of the current minimum distance (hmin (ti)) and tolerance (you (t,)), the current minimum distance (h _min (ti)) is increased. The method according to claim 3, wherein, when two successive state determinations are non-contact and the value of the motion parameter is less than a maximum motion threshold (SMM), said tolerance (you (tj)) is increased without being able to be higher than a maximum tolerance threshold (tol _max ). 7. Method according to one of the preceding claims, wherein the utensil (U) is provided with a fixed magnetic element (EM) and the localization device (DL) used is provided with a network (R) of at least N triaxial magnetometers (M 1) mechanically interconnected without any degree of freedom to maintain a known relative position of these magnetometers (Μ,), N being an integer of at least 2. 8. System for continuously detecting a state of contact or non-contact between a tip (P) of a utensil (U) and a writing surface (SE) of a writing medium (SUE) of variable thickness disposed on a support surface (SA) of a locating device (DL); said system comprising a computer adapted to:  - determining, at a current time (t,), the position (Pos (ti)) of the tip (P) of the utensil (E) relative to the bearing surface (SA);  determining, at the current moment (t,), a distance (t) between the bearing surface (SA) and the tip (P), from said position (Pos (ti)) of the tip (P); - update the current value of a minimum distance (h _min (ti)) between the bearing surface (SA) and the writing surface (SE) with the distance (h (ti)) between the surface d the support (SA) and the tip (P) at the current instant (t,), when said distance ((tj)) is smaller than the current value of the minimum distance (h _min (ti)); determining, at the current instant (t), a movement parameter (PM (t,)) of the tip (P), representative of a ratio between the movement tangential to the writing surface (SE) and the normal movement at the writing surface (SE) between the current instant (t,) and the previous instant (t, 1); determining, at the current instant (t,), an amplitude parameter (dPos (ti)) representative of the amplitude of the movement of the tip (P) between the current instant (t,) and the previous instant (ΐ, -ι); determining a state of contact between the tip (P) and the writing surface (SE), at the current moment (t,), when: the distance ((t,)) is less than or equal to the sum of a current minimum distance (h _min (ti)) and a tolerance (tol (ti)), said tolerance being representative of an uncertainty on the distance ((t,)) between the writing surface (SE) and the bearing surface (SA);  the motion parameter (ΡΜ (ΐ,)) is representative of a movement substantially tangential to the writing surface (SE), i.e. its value is less than or equal to a tangential motion threshold (SMT); and  a norm of the amplitude parameter (dPos (ti)) is greater than or equal to a threshold of minimum amplitude (dPOSmin); to determine, otherwise, a state of non-contact between the tip (P) and the writing surface (SE), at the current instant (.,).